Japanese Non-examined Patent Publication No. 2000-292522 discloses an antenna unit for GPS (Global Positioning System). This antenna unit is adapted to be inserted into an electronic device, such as a PDA and a notebook, and receives a satellite signal from a GPS satellite and transmits data, such as an actual location, to the electronic device. In this antenna unit, the distance from the electronic device to an antenna built into this antenna unit can be extended so as to reduce the possibility that noise generated at a CPU, etc. of the electronic device is propagated through midair and affects the antenna unit.
By the way, the noise transmitted from the electronic device to the antenna unit includes noise transmitted from the device to the unit through a ground line that connects between a ground of the electronic device and that of the antenna unit, in addition to the noise propagated through midair, as mentioned above. If the noise of the ground line is transmitted to the antenna unit, it may reduce the receiver sensitivity of the antenna unit because it may destabilize a ground level and destroy the waveform of the received radio signal. Although the above-mentioned antenna unit can reduce the noise propagated through midair, it can not reduce the noise transmitted through the ground line.